ANOMALOUS BEHAVIOR IN THE SPIN POLARIZATION OF LOW-ENERGY SECONDARY ELECTRONS FROM GD(0001)

We have measured the spin-polarization spectrum of low-energy secondary electrons from the surface of thick single-crystalline Gd(0001) films grown on W(110). Below 1.5 eV an anomalous decrease in the polarization is observed and is attributed to the presence of the 4f8 states just above the vacuum level, which leads to increased emission of spin-down electrons. Between 1.5 and 7 eV the spin polarization remains nearly constant at about 33% (at 150 K), demonstrating that the phenomenon of polarization enhancement for low-energy secondary electrons is not universal. The observed large polarizations indicate that the completely polarized 4f electrons contribute significantly to the secondary-electron yield.